1. Field of the Invention
The present invention relates generally to gas bearing turbocompressors. More particularly, the invention concerns turbocompressors embodying gas bearings wherein large temperature differences develop between the turbine and the compressor ends of the device, such as is the case in turbochargers for internal combustion engines where the turbine is driven by hot exhaust gases and the compressor receives cool intake air.
2. Discussion of the Invention
Turbocompressors have been in use as turbochargers for internal combustion engines for many years. These types of turbocompressors have generally embodied oil-lubricated bearings. The trend in recent years in turbocompressor design has been towards higher compression ratios requiring higher revolutions per minute (RPM) of the spindle of the turbocompressor and resulting in higher exhaust temperatures. Under such conditions oil lubrication of the bearings becomes inadequate and can possibly result in cavitation in the bearings as a result of the higher rubbing speed, and in thermal decomposition as a consequence of the higher temperatures. A solution to the aforementioned problems is provided by the use of gas bearings such as the bearings disclosed in U.S. Pat. No. 4,808,070 issued to the present inventor. The novel gas bearings disclosed in U.S. Pat. No. 4,808,070 can easily handle the required RPM and rubbing speeds of most types of modern turbo compressors.
In a typical turbocharger the compressor is fed through a filter which causes a significant pressure drop at the inlet to the compressor resulting in an inlet pressure lower than the atmosphere. On the other hand, the conventional turbine discharges to atmosphere through a muffler or catalytic converter also causing a pressure drop which results in a pressure higher than atmosphere at the exhaust of the turbine. The resulting pressure difference between the compressor housing and the turbine housing causes a net thrust to develop in the shaft connecting the compressor wheel and the turbine wheel. In an oil-lubricated turbo-charger this thrust is absorbed by a traditional oil-lubricated thrust washer. However, in the case of gas bearings the thrust is typically compensated by the arrangement described in U.S. Pat. No. 5,567,129 issued to the present inventor. Because of the relevance of the U.S. Pat. Nos. 4,808,070 and 5,567,129 to a complete understanding of the present invention, both of these patents are hereby incorporated by reference as though fully set forth herein.
As previously mentioned, the thrust of the present invention is directed towards solving the problems caused by large temperature differences between the ends of the turbocompressor. In this regard, the correct operation of the thrust bearings discussed in U.S. Pat. No. 5,567,129 depends in large measure on maintaining the correct clearances in the gaps between the stationary and moving parts of the thrust bearings. These gap dimensions are affected by changes in length of the housing and the length of the shaft due to thermal expansion. Such changes are particularly large if the temperature distribution in the housing differs appreciably from the temperature distribution in the shaft and the thermal expansion coefficient of the shaft differs substantially from that of the housing. These conditions result in serious operating problems. For example, if the shaft elongates more than the housing, the gaps increase thus decreasing the stiffness of the thrust bearings and increasing the parasitic mass flow through the bearings. Similarly, if the housing elongates more than the shaft the gaps may completely close resulting in contact of the facing surfaces of the adjoining components thereby resulting in severe damage to the bearings.
Other problems can occur if the gas flowing in the compressor is not compatible with gas flowing in the turbine. This latter problem can be solved by supplying gas to the thrust bearings from an independent source, which gas is compatible with the gas flowing in the turbine and compressor.
Although this invention is presented primarily in the context of turbochargers for use with internal combustion engines, the arrangement of the component parts also solves a different problem of great importance in the context of turbo-compressors for supplying air to fuel cells (see for example U.S. Pat. No. 5,523,176 issued to the present inventor). This problem concerns the danger of oil contamination in the supplied air, which contamination can poison the catalyst in the fuel cell. The turbocompressor of the present invention also permits the use of a buffer gas where any mixing of the gasses in the expander and compressor housings must be positively prevented.